A circuit known as a flyback converter is a switch mode power supply circuit commonly used in applications such as AC-to-DC wall adapter power supplies and battery chargers. FIG. 1 (Prior Art) is a block diagram of a simple flyback converter 1 that is controlled from the primary side of the power converter transformer and that operates in an emitter switching configuration. Flyback converter 1 includes a transformer 100 which has primary, secondary, and auxiliary windings. On the primary side, flyback converter 1 includes start-up resistor 101, capacitor 102, rectifier 103, switcher circuit 104, an NPN bipolar transistor switch 105, diode 106, and resistor 109. On the secondary side, flyback converter 1 includes a secondary rectifier 122 and an output capacitor 124. An alternating current (AC) line voltage may, for example, be rectified by a full wave bridge rectifier (not shown) and an associated smoothing capacitor (not shown) so that the rectified and smoothed DC voltage is present at terminal Vin.
Flyback converter 1 operates by repeatedly turning on and off NPN transistor 105. Turning on NPN transistor 105 causes a current to flow from a first input node Vin, through a primary side winding (having Np number of turns) of a transformer 100, through NPN transistor 105, and into SW terminal of switcher circuit 104. Turning off NPN transistor 105 causes the voltage at the dot end of the primary winding, and hence, the secondary side winding (having Ns number of turns) to swing positive, which causes current to flow through the rectifier diode 122 and into capacitor 124 to produce a DC output voltage at terminal Vout. Switcher circuit 104 controls the turning on and off of NPN transistor 105 from the emitter terminal of NPN transistor 105 by opening and closing a switch, which is connected between terminals SW and GND. An optocoupler circuit (not shown) provides feedback from the secondary side of transformer 200 to terminal FB of switcher circuit 104 to regulate the output voltage Vout.
The start-up time of flyback converter 1 is from the time a valid input voltage is present, Vin, which is greater than a minimum input voltage, to a time when a desired regulated output voltage, VREG, is provided at output voltage terminal Vout. During start-up, the voltage at terminal Vout rises from zero volts to the desired regulated output voltage, VREG. The start-up time depends on the RC start-up time constant, which is the product of the resistance of resistor 101 and the magnitude of capacitance of capacitor 102. As will be shown, the resistance of resistor 101 and capacitance of capacitor 102 are chosen to be large, which results in a large RC time constant and a long start-up time.
According to certain energy conservation standards, power consumption during standby mode for low power chargers is limited to a maximum of 300 milliwatts. Standby mode is when an input voltage is present at terminal Vin but no device draws power from terminal Vout. The resistance of resistor 101 is large to minimize power consumption during standby mode because power consumption of resistor 101 depends inversely on the resistance of resistor 101. The power consumption of resistor 101 is represented as:P=(Vin−VC)2/R101 For a 265 V RMS AC input line, the voltage at terminal Vin is as high as 375 V DC. For example, when VC is 15 V and R101 is 1.5 megaohms, the power dissipation from resistor 101 is 86 milliwatts, which accounts for almost one third of the maximum power consumption permitted by flyback converter 1 during standby mode.
The use of capacitor 102 is illustrated with regard to the waveform diagram of FIG. 2. The turn-on threshold voltage (shown as “TURN-ON VOLTAGE”) is the voltage at which switcher circuit 104 starts turning on or off NPN transistor 105 whereas the turn-off threshold voltage (shown as “TURN-OFF VOLTAGE”) is the voltage at which switcher circuit 104 stops operating. For example, the turn-on threshold voltage is 20 V and the turn-off threshold voltage is 10 V. Capacitor 102 has a large capacitance value because capacitor 102 supplies power to switcher circuit 104 at terminal VC after terminal VC reaches the turn-on threshold voltage (time T2 in FIG. 2) until the output voltage, Vout, is large enough so that the auxiliary winding powers switcher circuit 104 (at time T3) while also maintaining the voltage at terminal VC above the turn-off threshold voltage. Were the capacitance of capacitor 102 not large enough, then after time T2, the voltage at terminal VC would fall to below the turn-off threshold voltage and switcher circuit 104 would turn-off thereby causing flyback converter 1 to malfunction. From time T2 to time T3, the output voltage Vout rises until time T3, when the auxiliary voltage, which is approximately Vout*Na/Ns (neglecting the voltage drop across rectifier diode 122), exceeds the voltage at terminal VC supplied by capacitor 102. From time T3 onward, the auxiliary winding supplies power to switcher circuit 104 through terminal VC. The voltage at terminal VC settles to a constant voltage of approximately VREG*Na/Ns because the output voltage, Vout, settles to a regulated output voltage, VREG.
Capacitor 102 also has a large capacitance value because capacitor 102 supplies current to the base terminal of NPN transistor 105 when NPN transistor 105 is turned-on. When VC is 15 V and the resistor 109 is 500 ohms, the base current to NPN transistor 105 is approximately 30 milliamperes. For a 60 kHz switching cycle, capacitor 102 supplies 30 milliamperes of base current during the on-time portion of the switching cycle.
The large values of resistance of resistor 101 and capacitance of capacitor 102 result in a large RC time constant and a long start-up time for switcher circuit 104. For example, when the resistor 101 is 1.5 megaohms, capacitor 102 is 10 microfarads, Vin DC is 155 V (from a 110 V AC input), and the turn-on threshold voltage is 20 V, then the time to turn-on switcher circuit 104 (i.e., the time to reach time T2) is represented approximately as:Tturn-on≈(R101*C102*VTURN-ON THRESHOLD)/Vin DCUsing the values supplied above, the time to turn-on the switcher circuit 104 is approximately 1.9 seconds. Unfortunately, in some applications, this long start-up time of a flyback converter 1 is not acceptable. Some applications require a startup time of less than 100 milliseconds. A market exists for switching regulator systems with lower start-up times.